Childhood amnesia

Childhood amnesia, also called infantile amnesia, is the inability of adults to retrieveepisodic memories before the age of 2–4 years, as well as the period before age 10 of which adults retain fewer memories than might otherwise be expected given the passage of time.[1] For the first 1–2 years of life, brain structures such as the limbic system, which holds the hippocampus and the amygdala and is involved in memory storage,[2] are not yet fully developed.[3] Research has demonstrated that children can remember events from before the age of 3–4 years, but that these memories decline as children get older.[4][5]

Research has shown that children have the capacity to remember events that happened to them from age 1 and before whilst they are still relatively young, but as they get older they tend to be unable to recall memories from their youngest years.[6] When the offset of childhood amnesia is defined as the age of first memory, then offset occurs around 3.5 years[7] though it can range from 2 to 5 years, depending on the memory retrieval method[8] and age of the respondent.[6] However, when the offset of childhood amnesia is defined as the age at which the majority of memories are personal recollections rather than known events, then offset occurs at approximately 4.5 years old.[8] This may be due to children's development of understanding and knowledge of their own memory.[9]

Changes in encoding, storage and retrieval of memories during early childhood are all important when considering childhood amnesia.[10] Research shows differences between gender and culture, which is implicated in the development of language. Childhood amnesia is particularly important to consider in regard to false memories and the development of the brain in early years. Proposed explanations of childhood amnesia are Freud's trauma theory, neurological development, development of the cognitive self, emotion and language.

Childhood amnesia was first formally reported by psychologist Caroline Miles in her article "A study of individual psychology", published in 1893 by the American Journal of Psychology.[11] Five years later, Henri and Henri published a survey showing that most respondents’ earliest recollections occurred between the ages of two and four.[12] In 1904 G. Stanley Hall noted the phenomenon in his book, Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education.[13] In 1910, Sigmund Freud offered one of the most famous and controversial descriptions and explanations of childhood amnesia. Using psychoanalytic theory, he postulated that early life events were repressed due to their inappropriately sexual nature. He asserted that childhood or infantile amnesia was a precursor to the ‘hysterical amnesia,’ or repression, presented by his adult patients.[14] Freud asked his patients to recall their earliest memories and found that they had difficulty remembering events from before the ages of 6–8.[15] Freud coined the term "infantile" or "childhood amnesia" and discussed this phenomenon in his Three Essays on the Theory of Sexuality.[15]

Psychologists differ in defining the offset of childhood amnesia. Some define it as the age from which a first memory can be retrieved, others the age at which memories change from general memories to more specific autobiographical events.[8] It is generally agreed there is no set age that people should be able to remember events from. The nature of the childhood event and how the person retrieves a memory can influence what can be recalled.[16] The amount of early childhood memories a person can recall also depends on how old they are when they are asked to remember.[6]

Many studies use cued recall to retrieve memories.[17] In its basic form, the experimenter gives the participant a word, and the participant responds with the first memory they think of associated with that word.[12] This method has generally estimated the age of offset at approximately 5 years.[8] However, there are several objections to the cue method.[8] One memory is recorded per cue word, so it can be difficult to know whether this memory is their earliest memory or the first memory that popped to mind. It may be a problem if participants are not asked to record the earliest memory they can recall which relates to the cue.[8] If the experimenter asks the participant to specifically use childhood memories or the earliest memories associated with a cue, the age estimate lowers to 3–4 years.[12] Even with this measure, cued recall is only useful for bringing to mind memories formed several months after the introduction of that word into the participant's vocabulary.[18] One study performed by Bauer and Larkina (2013) used cued recall by asking children and adults to state a personal memory related to the word and then state the earliest time that it occurred. The researchers found that the younger children need more prompts or cues. However, for both children and adults, the earliest memory retrieval was around three years old.[19]

Free recall is the process by which experimenters ask individuals for their earliest memories, and allow the participant to respond freely.[12] This method is more accurate than the basic cued recall method,[8] and elicits memories from an earlier age. But there is no significant difference when people are instructed to recall their earliest memories with cued recall or earliest memories with free recall.[12] One major benefit of free recall is that every question gets answered.[12]

In the exhaustive recall method, participants are asked to record all the memories they can access before a specific age.[12] This method, like free recall, relies on participants to come up with memories without cues. Exhaustive recall yields a better understanding than others on the amount of memories surviving from early childhood, but can be demanding for the subjects who often have to spend many hours trying to remember events from their childhood.[12] No major differences among word cued, interview, focused and exhaustive recall have been found.[20]

Certain memories from the early childhood years are more accessible than others for adults. Although people assume that not recalling a childhood memory means they have forgotten the event, there is a difference between availability and accessibility. A memory may always be available, but a person might not be able to access the memory for various reasons.[12]

According to a study by West and Bauer, earlier memories have less emotional content than later memories, and are less personally meaningful, unique, or intense. Earlier memories also do not seem to differ greatly in perspective.[12][21][22] However, certain life events do result in clearer and earlier memories. Adults find it easier to remember personal, rather than public, event memories from early childhood.[8] This means a person would remember getting a dog, but not the appearance of Halley's Comet. Research has suggested that public events are remembered from approximately age 6.[12] Adults can remember salient events such as a hospitalization and the birth of a sibling earlier than most events (2 and 3 years, as opposed to 3.5 years). Other seemingly important events, like the death of a loved one and moving from one home to another, do not seem to be recalled as early.[23]

Research suggests that until around the age of 4, children cannot form context-rich memories. They cannot remember the source for given information because their prefrontal cortex is underdeveloped.[24] Adults can access fragment memories (isolated moments without context, often remembered as images, behaviors, or emotions) from around age 3.5, whereas event memories are usually recalled from slightly later.[25] This is similar to research showing the difference between personal recollections and known events. Known memories change to more personal recollections at approximately 4.5 years old.[8]

Children can form memories at younger ages than adults can recall. While the efficiency of encoding and storage processes allows older children to remember more,[26] younger children also have great memory capacity. Infants can remember the actions of sequences, the objects used to produce them, and the order in which the actions unfold, suggesting that they possess the precursors necessary for autobiographical memory.[11] Children's recall is 50% accurate for events that happened before the age of two[5] whereas adults remember near to nothing before that age.[4][8] By age two, children can retrieve memories after several weeks, indicating that these memories could become relatively enduring and could explain why some people have memories from this young.[4][11] Children also show an ability to nonverbally recall events that occurred before they had the vocabulary to describe them, whereas adults do not.[18][27][28] Findings such as these prompted research into when and why people lose these previously accessible memories.

It has been found that as children age, they lose the ability to recall preverbal memories.[29] One explanation for this maintains that after developing linguistic skills, memories that were not encoded verbally get lost within the mind.[29] This theory also explains why many individuals' early memories are fragmented – the nonverbal components were lost.[30] However, contrary findings indicate that elementary aged children remember a greater amount of accurate details about events than they had reported at a younger age[31] and that 6–9 year old children tend to have verbally accessible memories from very early childhood.[31]

This increased ability for children to remember their early years does not start to fade until children reach double digits.[30] By the age of eleven, children exhibit young adult levels of childhood amnesia.[30] These findings may indicate that there is some aspect of the adolescent brain, or the neurobiological processes of adolescence, that prompts the development of childhood amnesia.

Generally, when a gender discrepancy is found in the age at first memories, females have earlier memories than males.[32] Women's earlier first memories may be accounted for by the fact that mothers generally have more elaborative, evaluative, and emotional reminiscent style with daughters than with sons,[33] which has been shown to result in more richly detailed childhood memories.[33] Women across cultures tend to have more information-dense memories than men,[32] and women tend to refer to others more than themselves in their earliest memories.[32][34] Men, on the other hand, exhibit more early memory focus on their individual selves. Men have been found more likely than women to mention negative memories.[32] Contrarily, studies have shown that girls are more likely to remember traumatic and transitional events, whereas boys more often remember play events.[30] Early recollections have also been found more accurate in their reflections of friendliness for men and dominance for women.[35]

MacDonald et al. found that Chinese participants had later first memories than New Zealand European (Pakeha) or Maori participants. This effect was due to Chinese females, whose average age at first memory was 6.1 years.[32] This indicates that Chinese women have later first memories than Chinese men, which differs from the general finding that women report earlier first memories than men. It has been suggested that since sons are prized far over daughters in China, parents may have more elaborate, evaluative, and emotional reminiscent styles with boys than with girls.[32] Among American subjects, it has been found that Black women have later memories than Black males or White females.[34] Black women also tend to report a low proportion of personal experience[34] which is independently correlated with being older at the age of first memory. It may be that with parents are more likely to use directives than to elaborately reminisce with daughters in Black American culture.[34]

Findings that Korean individuals have significantly later first memories than American individuals was originally thought to be caused by the collectivist nature of Asian cultures.[32] The lack of an age discrepancy between Chinese males and New Zealand European individuals serves to cast doubt on this theory.[32] Additionally, studies on the Black American population, which is considered a more collectivist society, have not indicated later first memories than non-collectivist cultures.[34] It has been shown, however, that children from Western cultures tell more elaborated, detailed, and emotional narratives than children from eastern cultures.[33]

Maori adults report significantly earlier memories than either Pakeha or Chinese individuals. The traditional emphasis on the past in Maori culture may have led to an earlier understanding of the nature of time, and to the retrieval of earlier memories. Maori are also more likely than Pakeha or Chinese individuals to indicate a family story as a source for their memory.[32]

Individuals' first memories significantly reflect their personality traits.[36] People who reveal a more detailed memory are more likely to be open in their daily lives and disclose a range of personal information to others.[35] Characteristics of early recollections are reflective of friendliness for males and dominance for females.[35]

Even when childhood events are not remembered episodically, they can be implicitly remembered. Humans can be primed and implicitly trained earlier before they can remember facts or autobiographical events.[24] This is most important in terms of emotional trauma. Adults can generally recall events from around 3.5 years,[7] and have primarily experiential memories beginning around 4.7 years old.[8] However, adults who had traumatic and abusive early childhoods report an offset of childhood amnesia around 5–7 years old.[7] It has been suggested that this is because stressful experiences can injure memory centers and possibly make it harder to form memories.[7] This coupled with the fact that priming can occur at a younger age may indicate that children in abusive situations have implicit memory connections that were formed in response to the abuse.[7][14][24] Whether or not these 'repressed' memories can affect individuals is a matter of considerable debate in psychology.

Very few adults have memories from before 2.5 years old. Those who do report memories from before this age usually cannot tell the difference between personal memory of the event and simple knowledge of it, which may have come from other sources.[37] Events from after the age of 10 years are relatively easy to remember correctly, whereas memories from the age of 2 are more often confounded with false images and memories.[38] Memories from early childhood (around age two) are susceptible to false suggestion, making them less trustworthy. These should be treated with caution, especially if they have severe consequences.[38] Imagining details of a false event can encourage the generation of false memories.[38] Studies have shown that people who merely imagine a childhood event are more likely to believe that it happened to them compared to events they did not imagine.[39] This term has been coined imagination inflation and shows that merely imagining an event can make it seem more plausible that it really happened. Using the same paradigm, people that are shown a doctored photograph of themselves as a child in an event that never occurred can create false memories of the event by imagining the event over time.[40] Therefore, this implies that it would be possible for false memories to be generated in and/or fed by a court case. This concern has led the APA to advise caution in accepting memories of physically and sexually abusive events from before the age of 2. However they also recommend that these memories not be entirely discounted, due to the heinous nature of the crimes.[1]

Sigmund Freud is famous for his theories of psychosexual development which suggest that people's personality traits stem from their libido (sexual appetite) which develops from early childhood experiences.[14] Freud theorized that childhood amnesia was the result of the mind’s attempt to repress memories of traumatic events that occurred in the psychosexual development of every child. This supposedly led to the repression of the majority of memories from the first years of life when children were supposedly obsessed with exploring their sexuality.[14]

Freudian theory, including his explanation for childhood amnesia, has been criticized for extensive use of anecdotal evidence rather than scientific research, and his observations that allow for multiple interpretations.[41] Examining the effects of emotional trauma and childhood amnesia shows that stressful experiences do in fact disrupt memory and can damage central parts of the memory system such as the hippocampus and amygdala.[7] Adults who were abused or traumatized in childhood form their earliest memories about 2–3 years after the general population. In addition, they demonstrate considerable problems with visual, pictorial, and facial memory storage and retrieval compared to non-traumatized individuals.[7] This implies that trauma can disrupt the formation of early childhood memories, but does not necessarily give evidence for Freud's theory of repression.

The amygdala (which is primarily concerned with emotions and emotional content of memories) and the hippocampus (which concerns primarily autobiographical memories) are generally independent, but emotions and the amygdala are known to play a role in memory encoding, which is typically associated with the hippocampus.[2] Research has found that later memories in childhood have more propositional and emotional content than earlier memories and are rated as more meaningful and vivid. It has been suggested that differences in the emotions experienced by infants and adults may be a cause of childhood amnesia.[21] Whether highly emotional events can stimulate and improve reliable recall (flashbulb memories) is still highly debated.[42]

Studies have discovered that emotional experiences are connected with faster retrieval times, leading to the belief that emotional events have heightened accessibility in our memories.[1] If an event is particularly surprising, it receives prioritized processing in the brain, most likely due to evolutionary reasons. The evolutionary theory states that if a past event was particularly frightening or upsetting, one is apt to avoid a similar situation in the future, especially if it is endangering to one's well-being. In addition, the more significant an event, the bigger impact it has and the more rehearsal it receives.[1]

Findings have shown that events such as hospitalization and the birth of a sibling are correlated with an earlier offset of childhood amnesia, which may be because they were more emotionally memorable.[23] However, other seemingly emotional memories such as the death of a family member or having to move do not affect offset, possibly because the events were not as meaningful to the child.[23] Some memories are therefore available from earlier in childhood than others, which has led to the conclusion that very emotional events can be encoded and recalled earlier than non-emotional events.[23]

One possible explanation for childhood amnesia is the lack of neurological development of the infant brain, preventing the creation of long term or autobiographical memories.[1] The hippocampus and prefrontal cortex, two key structures in the neuroanatomy of memory, do not develop into mature structures until around the age of three or four. These structures are known to be associated with the formation of autobiographical memories.[24][41]

The physiological approach appears to support findings about memory loss in relation to amnesiacs and others who have experienced damage to the hippocampus. They cannot efficiently store or recall memories from past events, but still exhibit perceptual and cognitive skills and can still learn new information.[1] The development of the Medial Temporal Lobe (MTL), which contains the hippocampus, has been found to specifically have a defining impact on the ability to encode and maintain memories from early childhood.[43]

While the neurological explanation does account for blanks in very young children’s memories, it does not give a full explanation for childhood amnesia because it fails to account for the years after the age of four. It also fails to address the issue that children themselves do not show childhood amnesia.[1] Children around the age of two to three have been found to remember things that occurred when they were only one to two years old.[28] This discovery that three-year-olds can retrieve memories from earlier in their life implies that all necessary neurological structures are in place.[44]

There are, however, reasons to believe that different associations within the cerebral hemisphere have an effect on remembering events from a very early period in a person's life. Mixed-handedness and bilateral saccadic eye movements (as opposed to vertical or pursuit eye movements) have been associated with an earlier offset of childhood amnesia, leading to the conclusion that interactions between the two hemispheres correlate with increased memory for early childhood events.[45]

The development of a cognitive self is also thought to have a strong effect on encoding and storing early memories.[1] As toddlers grow, a developing sense of the self begins to emerge as they realize that they are a person with unique and defining characteristics and have individual thoughts and feelings separate from others. As they gain a sense of the self, they can begin to organize autobiographical experiences and retain memories of past events. This is also known as the development of a theory of mind which refers to a child’s acceptance that they have beliefs, knowledge, and thoughts that no one else has access to.[1]

The developmental explanation asserts that young children have a good concept of semantic information, but lack the retrieval processes necessary to link past and present episodic events to create an autobiographical self.[24] Young children do not seem to have a sense of a continuous self over time until they develop awareness for themselves as an individual human being. This awareness is thought to form around the age of 4 or 5, as children in this time period can understand that recent past events affect the present, while 3-year old children still seem unable to grasp this concept.[46]

This acknowledged link of the past to the present and the concept of continuous time and therefore a continuous self is also helped by memory talk with adults.[1] Through elaboration and repetition of events experienced, adults help children to encode memories as a part of their personal past and it becomes essential to their being.[1]

The incomplete development of language in young children is thought to be a critical cause of childhood amnesia as infants do not yet have the language capacity necessary to encode autobiographical memories.[1] The typical schedule of language development seems to support this theory. At the age of one, babies tend to be limited to one word utterances, and childhood amnesia predicts that adults have very few, if any, memories of this time. By the age of three, children are capable of two or three word phrases, and by age five their speech already resembles adult speech.[41] There appears to be a direct correlation between the development of language in children, and the earliest age at which we can obtain childhood memories (around the age of four).[41] Performance on both verbal and nonverbal memory tasks shows that children with more advanced language abilities can report more during a verbal interview and exhibit superior nonverbal memory compared to children with less advanced language skills.[27] If children lack language, they are unable to describe memories from infancy because they do not have the words and knowledge to explain them. Adults and children can often remember memories from around three or four years of age which is during a time of rapid language development. Before language develops, children often only hold preverbal memories and may use symbols to represent them. Therefore, once language comes online, one can actively describe their memories with words. The context that one is in when they encode or retrieve memories is different for adults and infants because language is not present during infancy.[47]

Language allows children to organize personal past and present experiences and share these memories with others.[1] This exchange of dialogue makes children aware of their personal past and encourages them to think about their cognitive self and how past activities have affected them in the present.[33] Several studies have shown that simply discussing events with children in general could lead to more easily retrievable memories.[4] There has also been research that suggests the degree to which a child discusses events with adults shapes autobiographical recollection.[48] This has implications for gender and cultural differences. Autobiographical memory begins to emerge as parents engage in memory talk with their children and encourage them to think about why a certain event happened. Memory talk allows children to develop memory systems in order to categorize generic versus unique events.[1]

The social cultural developmental perspective states that both language and culture play a role in the development of a child's autobiographical memory.[49] An important aspect of this theory considers the difference between parents who discuss memories at length with their children in an elaborative style, and those who do not. Children of parents who discuss memories with them in an elaborative style report a greater number of memories than children who do not discuss their memories. Memories are described with greater detailed. This has implications for cultural differences.

^ abBauer, Patricia; Jennifer Wenner; Patricia Dropik; Sandi Wewerka (August 2000). "Parameters of remembering and forgetting in the transition from infancy to early childhood". Monographs of the Society for Research in Child Development65 (4): v–204.